JUNE 22, 1899] 
NATURE 
175 
Habits of the Cuckoo. 
One day last week I was in my garden—a not particularly 
private country one—when I heard a cuckoo close by, and, 
‘standing quite still, I saw the bird alight upon an apple tree 
not more than four yards from me. The bird did not appear 
to object to my close proximity, for it uttered its call ‘* cuckoo ” 
twice. Its mate then came and sat in a plum tree only five 
yards from me, on the opposite side of me; one of them had a 
caterpillar in its mouth. Then a blackbird came into another 
tree in a state of great excitement uttering its ‘‘ pink pink,” as 
I supposed, at the cuckoos ; and the question arose in my mind, 
““Does the cuckoo feed its own young, and was that in the 
blackbird’s nest?” Can any of your readers help me? 
Wn. H. WILSON. 
Gloucester House, Sudbury, Harrow, June 19. 
Economic Entomology. 
Can you tell me where I can get information as to the 
present condition of economic entomology in this country, 
‘more especially as to methods of research usually adopted ? 
MAGNETIC PERTURBATIONS OF THE 
SPECTRAL LINES. 
ope subject which we are about to consider this even- 
ing forms a connecting link between two of the 
most interesting branches of human knowledge—namely, 
that which treats of magnetism and that which treats of 
light. Almost as soon as the properties of magnets 
became known, mere curiosity alone must have prompted 
philosophers to ascertain if any relation existed between 
magnetism and “the other forces of nature,” as they 
were generally termed. We are consequently led to 
expect amongst the records of early experimental in- 
vestigations some accounts which treat of the action of 
magnetism on light. 
Early Experiments. 
When we seek for such accounts, however, we find that 
they are almost wholly absent from the literature of 
science, and this arises, I believe, from the great difficulty 
of the investigation and from the circumstance that only 
negative results were obtained, rather than that no such 
inquiry suggested itself or was undertaken. Even in 
-quite recent times this inquiry has been prosecuted, but 
without success, by physicists who have published no 
account of their experiments. We may take it, there- 
fore, that the inquiry is in itself an old one, although it 
is only now that it has been carried to a successful issue. 
The earliest recorded attempt to solve this problem 
with which we are acquainted, is that of a celebrated 
British physicist whose name must for ever shed lustre 
on the annals of the Royal Institution—I speak of 
Michael Faraday. In order to understand the nature of 
the investigation which Faraday took in hand, and which 
has led up to the discourse of this evening, it is best to 
consider briefly some elementary facts concerning mag- 
netism and light. 
Magnetic Field of Force. 
In the first place, I shall assume that we know ina 
general way what the peculiarities of a body are which 
lead us to say that it is magnetised, or a magnet. These 
are that, when freely suspended, it sets itself in a definite 
direction over the earth’s surface, as illustrated by the 
compass needle, and that in the space around it there is 
“magnetic” force exerted on pieces of iron, and, in a 
smaller degree, on othersubstances. For this reason, we 
say that a magnet is surrounded by a magnetic field of 
force. The field of force is simply the space surrounding 
‘the magnet, and it extends to infinity in all directions 
1 Friday evening discourse delivered at the Royal Institution, May 12, 
iby Thomas Preston, M.A., D.Sc., F.R.S. 
NO. 1547, VOL. 60] 
from the magnet. Near the magnet the force is strong, 
and far away from it the force is almost insensible ; and 
so we say that the field is strong at certain places near 
the magnet, and that it is weak at places far away from 
the magnet. The direction of the force at any point is 
the direction in which the north pole of another magnet 
would be urged if placed at that point, and the push which 
this pole experiences may be taken to represent the 
intensity or strength of the magnetic field at the point in 
question. This is represented diagrammatically by these 
drawings [referring to figures suspended before the 
audience], which show roughly the nature of the field of 
force surrounding an ordinary bar magnet, a horse-shoe 
magnet, and the much more powerful form—the electro- 
magnet. It will be seen that the space outside the iron 
is filled with a system of curved lines running from the 
north pole to the south pole of the iron core. Where the 
lines are closest together there the magnetic force is 
strongest, and the direction of a line at any point is the 
direction of the resultant magnetic force at that point— 
that is, the direction in which a north pole would be urged 
if placed at that point. 
Faraday always pictured the magnetic field as filled 
with lines of force in this way, and the importance of 
the conception can scarcely be over-rated, for it leads 
us to view the magnetic action as being transmitted 
continuously through the intervention of some medium 
filling all space, rather than by the unintelligible pro- 
cess of direct action at a distance. This medium is 
called the ether; but as to what it is that is actually 
going on in the ether around a magnet, we cannot 
definitely say. It may be that there is a flow of ether 
along the lines of magnetic force, so that there is an out- 
flow at one end of the magnet and an in-flow at the 
other, or it may be that the ether is spinning round the 
lines of force in the magnetic field. For our present pur- 
pose, it is nota matter of very much importance what the 
exact condition of the ether may be in a magnetic field, 
for if the ether in a magnetic field is either in some 
peculiar condition of strain or of motion, and if light con- 
sists of an undulatory motion propagated through this 
same ether, then it may be naturally expected that some 
action should take place when light is propagated 
through, or radiated in, a magnetic field of force. This 
is what Faraday suspected, and in order that we may 
appreciate the problem with which he had to deal, let us 
place ourselves in his position and ask ourselves the 
question: ‘In what manner can we test experimentally 
if there is any magnetic action on light ?” 
Tests for Magnetic Action on Light. 
In answer to this question, the first thing that occurs to 
us is to pass a beam of ordinary light through the 
magnetic field, in some chosen directions, and examine 
by all the means at our disposal if any action has taken 
place. When this is done we find that no observable 
effect is produced. But the scientific investigator does 
not rest satisfied with one negative result. He varies 
the conditions of the experiment, and returns to the 
attack with renewed vigour and hopes. In our first trial 
we passed a beam of light through the air-filled space 
around the magnet, and we may vary this experiment 
either by removing the air altogether, and so causing the 
beam to traverse a vacuum, or we may replace the air 
by some dense transparent substance such as glass or 
water. Under these new conditions, we still fail to detect 
any influence of the magnetic field on a beam of ordinary 
light. This negative result might arise from the field of 
force being too weak to produce an observable effect, or 
it might be that the effect, if any effect really does exist, 
may be of such a character that it is impossible to detect 
it with ordinary light. In common light, the vibrations 
take place indifferently in all directions around the ray, 
and follow no law or order as to their type. They 
